To obtain accurate channel state information at the user equipment (UE) the Evolved Universal Terrestrial Radio Access (E-UTRA) LTE-Advanced standard employs a new reference symbol called Channel State Information-Reference Symbol (CSI-RS).
CSI-RS transmission is required for channel estimation at the UE since Rel. 8 Cell-specific Reference Symbols (CRS) are available only up to 4 antenna ports. For 4 transmitting antennas (TX) CRS is transmitted on antenna ports 0, 1, 2 and 3). A general principle of Rel. 10 reference symbols (RS) is precoded UE-RS used for data demodulation for up to an 8 layer transmission while non-precoded cell-specific CSI-RS is used for link adaptation. Existing Rel. 8 CRS will be used for control and measurement purposes and providing backwards compatibility for channel estimation at Rel. 8 UEs.
It is important to design CSI-RS reference patterns corresponding to the OFDM time-frequency resource block grid to minimize interference while measuring channel quality information (CQI) at a given UE from CSI-RS transmissions from interfering multiple cells.
The E-UTRA LTE-Advanced standard currently includes the following:
CSI-RS port multiplexing is based on Code Division Multiplexing (CDM) for each pair of CSI-RS ports.
Avoidance of port 5 of the same cell.
Two, 4 and 8 CSI-RS ports are nested to simplify implementation. This means that pattern with smaller number of CSI-RS ports (such as 1, 2 or 4) is a subset of the pattern with larger number of CSI-RS ports (2, 4 or 8).
A time-invariant time/frequency shift is used for CSI-RS transmission in a cell.
The following high-level design guidelines are advantageous the final CSI-RS pattern:
Minimizing Rel. 8 UE Performance Degradation.
Because Rel. 8 LTE UEs are oblivious to CSI-RS puncturing of their PDSCH REs, these UEs will experience throughput degradation and a Block Error Rate (BLER) floor at high Signal to Noise Ratio (SNR) especially when scheduled with a higher-order Modulation and Coding Scheme (MCS). Appropriate eNodeB scheduling of such UEs on non CSI-RS subframes may minimize the CSI-RS footprint across subframes. Alternately MCS downscaling such as Quadrature Phase Shift Keying (QPSK) ½ may be used whenever Rel. 8 UEs are scheduled in CSI-RS subframes.
Subframe Indices in FDD Systems.
CSI-RS transmissions should not occur in Physical Broadcast CHannel (PBCH), Synchronization Channel (SCH) or Paging channel carrying subframes to avoid problems with cell-search and initial acquisition.
Channel Estimator Complexity.
UE channel estimator complexity should be minimized by: using a CSI-RS pattern completely specified with just one resource block that is shift-invariant across frequency domain; Nested property whenever number of eNodeB APs is less than 8; CSI-RS RE locations for AP i>0 should be implicitly determined by the Orthogonal Frequency Division Multiplexing (OFDM) symbol and resource element position of AP 0; to ease channel estimation, all APs corresponding to CSI-RS in a given cell should be transmitted within a single subframe.
Interference Avoidance on Physical Downlink Control CHannel (PDCCH).
To avoid collision with PDCCH for any control region size, CSI-RS should not be carried on OFDM symbols 0 through 2. OFDM symbol 3 should be used to transmit CSI-RS as long as the operational bandwidth is greater than 1.4 MHz for higher re-use considering the limited availability of real-estate.
Interference Avoidance on CRS.
To ensure that Rel. 8 UE can receive CRS at least on APs 0 and 1, the CSI-RS transmissions should not occur on OFDM symbols {0, 4, 7, 11} in normal Cyclic Prefix (CP) operation and OFDM symbols {0, 3, 6, 9} in extended CP operation.
Interference Avoidance on UE-RS.
Because Rel. 10 UEs uses UE-RS for data demodulation, CSI-RS transmissions should not occur on REs positions intended for Rel. 10 UE-RS transmissions.
Interference Avoidance on Rel. 8 DM-RS.
For accurate channel estimation over Rel. 8 DM-RS (AP 5) at UEs configured for transmission mode 7, CSI-RS transmissions should not puncture out DM-RS REs. There are six options. In the first option eNodeB avoids AP 5 by picking a reuse pattern located outside of the AP 5 OFDM symbol. This permits the eNodeB to avoid collision with AP 5, but may result in a smaller reuse factor, especially when RS corresponding to APs 2, 3 and 5 are transmitted in a CSI-RS subframe. In the second option the eNodeB avoids AP 5 by picking a reuse pattern for a given cell specific Rel. 8 DM-RS shift, if possible. This avoids causing interference on AP 5, while ensuring a reuse factor which is at least as large as those obtained using the first option. The third option avoids AP 5 by scheduling whereby AP 5 is never sent in a subframe with CSI-RS. Thus the AP 5 and CSI-RS should not be shown in the same subframe. This option increases eNodeB scheduler complexity because it cannot configure a Rel. 8 UE for TM7 in CSI-RS subframes. In the fourth option the eNodeB transmits AP 5 RS and CSI-RS by superposition on the same RE. In the fifth option either AP 5 RS or 1 CSI-RS is transmitted, while the other one is punctured. Since CSI-RS is transparent to Rel. 8 UEs configured for Transparent Mode 7 (TM7), superposition of puncturing may result in inferior channel estimation quality at these UEs. This can affect data demodulation procedure. Further the RB selective transmission of CSI-RS of the fifth option may deteriorate intra-cell channel estimate due to the already low PRB density of CSI-RS of 1 RE/PRB/port. In the sixth option CSI-RS REs avoid RE positions corresponding to AP 5 if TM7 is enabled in a CSI-RS carrying subframe. If TM7 is not enabled with no UE is configured for TM7, then CSI-RS may reuse RE positions corresponding to AP 5. This sixth option requires one-bit signalling from eNodeB to Rel. 10 UEs to indicate whether TM7 is enabled in that CSI-RS subframe. This results in increased specification complexity. Whether TM7 is enabled may vary from one subframe to the next, implying significant signalling overhead when transmitting the one bit message.